Fish lay a large number of eggs because only a few of them, out of thousands, will be fertilized and produce a fish. This is because they are fertilized outside of the body and are highly sensitive to temperature changes and external factors.
Female salmon, for instance, will lay anywhere from 1,500 eggs to about 10,000 at one time. Among these eggs, only around 10 will successfully spawn salmon. The eggs themselves are highly sensitive, which can make it difficult for them to survive and hatch. Factors like increased pollution and warm temperatures can be contributing factors to the low success rate of fish eggs.